A typical ion source of the type with which the present invention is concerned comprises an ionisation, or arc, chamber (also sometimes referred to as a plasma chamber) into which source material can be introduced. A plasma is generated in the chamber in which the source material is ionised, and the ions are drawn off, by one or more extraction electrodes, through an outlet from the chamber. One typical ionisation chamber has a pair of cathodes arranged in opposed relationship, one referred to as a cathode and the other as a counter cathode. In use, the cathode is heated and emits electrons. The counter cathode then repels the electrons so that they are entrained between the two electrodes. The counter cathode may be connected to be at the same potential as the cathode or may be allowed to float.
The cathode and counter cathode are typically made of tungsten which has a high melting point (approx. 3700° K), and in some chambers, the interior surfaces of the chamber are formed of or coated with tungsten. Due to its high melting point, tungsten is normally unaffected by the temperatures generated within the chamber (c. 1000° C.) other than to emit electrons into the chamber to create, and sustain the plasma. The chamber is typically within a magnetic field which causes the electrons to move in fixed paths (e.g. spiral) between the cathode and counter cathode. Ionisation of the source material is achieved by the application of energy to the source material in a number of different ways, e.g. by applying an arc potential between the cathode and the chamber body, or in other types of ionisation chamber by use of r.f. or microwave energy. An extraction electrode which (for positive ions) is negatively biased relative to the chamber itself is mounted at the outlet from the chamber, typically outside the chamber. The negatively biased extraction electrode draws the ions through the outlet and the ions then pass through an aperture in the extraction electrode to the rest of the ion implantation apparatus.
Many different types of ions are implanted into substrates to impart required characteristics and properties to the resultant substrate.
Where it is desired to implant carbon or oxygen into a substrate, carbon dioxide has been selected as the most preferred source for carbon ions and is also suitable for oxygen ions, because carbon dioxide is plentiful, relatively safe to use and cheap to produce.
When producing carbon ions, it is also preferred to use carbon dioxide rather than carbon monoxide, even though ion yields would be higher from carbon monoxide, due to the known toxicity of the latter.
However, in using carbon dioxide in an arc chamber, it has been found that the actual presence of oxygen ions in the chamber reduces the useful life of the ion source due to the production of stoichiometric and non-stoichiometric oxides of tungsten at the electrodes and, where other tungsten surfaces are present in the chamber, at those surfaces as well. These oxides form within the chamber and it has been observed that the ion source becomes contaminated by the formation of these oxides and ceases to function efficiently within a few tens of hours. The oxides build up to form a barrier to the emission of electrons from the tungsten electrode and to conduction at the other tungsten surfaces of the chamber forming the anode. This effect can be regarded as ‘poisoning’ of the arc chamber, and occurs both when carbon ions are to be extracted from the ion source and when oxygen ions are to be extracted.
In U.S. Pat. No. 6,135,128 (Graf et al.), a method of cleaning an ion source comprising a plasma chamber has been proposed in which a dopant gas, selected from phosphine (PH3) containing phosphorus (P) arsine (AsH3) containing arsenic (As), germane (GeH4) and germanium tetrafluoride (GeF4) both containing germanium (Ge), and diborane (B2H6) containing boron (B), together with nitrogen trifluoride (NF3) as a cleaning gas, are introduced into the plasma chamber and ionised at temperatures of about 1000° C. The purpose of introducing the cleaning gas is to remove deposits of phosphorus or arsenic or germanium or boron, as the case may be, from interior surfaces of the plasma chamber when, without the presence of the cleaning gas, they can quite readily form such deposits on those surfaces. Where a plasma chamber is being operated to produce one or other of these species of ions alone, the matter of formation of deposits of the element in question is not problematic. However, typically, plasma chambers are used to produce ions of different species, and residue of one material on the interior walls of the plasma chamber will lead to impurities and contamination being present when creating an ion beam of another ion species.
The disclosure of Graf et al. is related to removal of depositable material from a plasma chamber. This can be achieved by varying the flow rate of the cleaning gas to remove any material which might be deposited as it is formed.